In large mixer bowl with electric mixer at medium speed, beat together butter, peanut butter, and sugars until very smooth and light in color.Beat in eggs and vanilla.In small bowl, combine flour and baking soda.Stir into peanut butter mixture until evenly mixed.Chill dough for 30 minutes.Roll walnut-sized pieces of dough between hands to make even balls.Roll in crushed Butterfingers and place on parchment-lined baking sheets.Bake in 3500 oven for 8 to 10 minutes until puffy (do not overbake).Cool 2 minutes on baking sheets,Cool completely on wire racks.Drizzle melted chocolate over cookies.Makes about 4 dozen.